1. Field of the Invention
The invention relates to an extensible driving sprocket for a bicycle transmission assembly.
2. Description of Background and Relevant Information
Extensible driving sprockets, or chainwheels, in bicycle transmission assemblies are known to perform a progressive gear shift.
For example, French Patent Publication FR 961 243 describes a drive sprocket having four pinions for driving the chain that are radially extensible.
For this type of construction, the chain follows a polygonal trajectory in the area of the drive sprocket, with substantial variations in distance from the axle of the sprocket which result in a corresponding substantial variation in the pedaling force.
French Patent Publications FR 1 140 134 and FR 1 542 594 describe another constructional embodiment, according to which the sprocket does not have pinions but instead radially extensible ring segments.
The drawback to these types of construction is controlling the transitory phases. It is readily understood that at the non-extensible state the segments are contiguous and that in their various states of extension, they leave between them a space that is equal to a whole number of the pitch of the chain. However, in the case of a radial extension, there is the drawback of controlling the space between two contiguous segments during the transitory phases, i.e., during the passage of the segments to the following phase of extension or retraction.
Another flaw in these known constructions is that they seek to multiply the number of transmission ratios, i.e., the number of positions for extending the segments. However, the further apart the sprocket segments are, the more difficult it is to change the segment positions, and the more perceptible is the previously mentioned phenomenon of polygonal trajectory.
Another flaw is that the mechanism for radially extending the segments is located toward the middle of the segment. It directly sustains the forces resulting from the tension of the chain. Furthermore, it is overhangingly stressed each time a segment is partially engaged with the chain. Added to this is the fact that the sprocket applies pressure to the taut strand of the chain, i.e., the strand that has the maximum stress.